The size you choose is a trade off between the amount of money you want to invest and the amount of electricity you want to produce.
A typical Austin home during peak electricity usage in the summer uses up to 5 kW of electricity at any given moment. Over the course of a year, that same home will use approximately 12,000 kWh of electricity.
You can look at your old electricity bills to calculate how much electricity your home typically uses. But remember, you do not need to cover all your electricity needs with a solar system to add value to your home.
The only other condition in deciding proper system size is the amount of suitable space available on your roof. To be suitable, you must have unobstructed, non-shaded roof area, either flat or a sloped, at an orientation other than North. You need roughly 100 square feet of roof space for each kWh kW that you wish to produce.